Evolutionary ecology of sex-determining mechanisms in reptiles

snapping turtleA fundamental problem in biology is understanding the ecology, genetics, and evolution of diverse sex-determining mechanisms. To address these questions, we have focused on reptiles as model organisms because they exhibit a tremendous diversity in modes of sex determination, including temperature-dependent sex determination (TSD) (Janzen and Paukstis, 1991, Quart. Rev. Biol.; Valenzuela et al., 2003, Am. Nat.; Janzen and Krenz, 2004). In the past we have studied the adaptive significance of TSD (Janzen and Paukstis, 1991, Evolution; Janzen, 1995, Evolution; Valenzuela and Janzen, 2001, Evol. Ecol. Res.; Janzen and Morjan, 2002, J. Herpetol.; Morjan and Janzen, 2003, Copeia; Janzen and Phillips, 2006, J. Evol. Biol.), the heritability of TSD in snapping turtles (Chelydra serpentina) (Janzen, 1992, Genetics), the levels and impacts of endogenous egg steroid hormones on sexual differentiation in five turtle species (Janzen et al., 1998, Gen. Comp. Endocrinol.), the influence of vegetation cover on sex ratios in natural nests of painted turtles (Chrysemys picta), which have TSD (Janzen, 1994, Ecology), and the effect of climatic variation on annual sex ratios produced in a natural population of Chrysemys (Janzen, 1994, PNAS). Current research on this topic includes studying nesting behavior in Chrysemys (Janzen and Morjan, 2001, Anim. Behav.; Harms et al., 2005, Physiol. Biochem. Zool.; Schwanz & Janzen 2008, Physiol. Biochem. Zool.) and in Chelydra (Kolbe and Janzen, 2002, Ecology), evaluating the physiological/endocrinological mechanisms underlying the adaptive significance of TSD in those two species (St. Juliana et al., 2004, Behav. Ecol. Sociobiol.), modeling the response of populations with TSD to predicted levels of global warming (Mitchell & Janzen, 2009, Sex. Dev.; Schwanz et al., 2010, Evolution), investigating the role of maternal effects on sexual differentiation in red-eared slider turtles (Trachemys scripta), and using comparative analyses to evaluate the possibility that TSD caused the extinction of dinosaurs at the end of Cretaceous.